Regulated DC power supplies are typically needed for most analog and digital electronic systems. Two major categories of regulated DC power supplies are linear power supplies and switching power supplies. Generally, in linear power supplies, a transistor (operating in its active region) is connected in series with an input voltage source and the voltage drop across the transistor is automatically adjusted to maintain an output voltage at a desired level.
In switching power supplies, transformation of DC voltage from one level to another is accomplished typically by means of DC/DC converter circuits, such as step-down (buck) or step-up (boost) converter circuits. Solid-state devices, such as transistors, are operated as switches (either completely ON or completely OFF) within these switching converters. Since the power devices are not required to operate in their active region, this mode of operation results in lower power dissipation. Furthermore, increasing switching speeds, higher voltage and current ratings of these power devices are some of the advantages that have increased the popularity of switching power supplies.
There are applications where a regulated output voltage of a power converter is required to vary between the minimum and maximum values of the power converter's input voltage supply, with no isolation needed. One example of such an application is in wireless communication base stations, wherein a signal amplifier typically requires a 28 volts regulated DC voltage off an input voltage source that varies from 20 volts to 30 volts. A preferred power converter topology for these applications is a step-down/up or buck-boost converter. The conventional buck-boost converter or its derived topologies are generally not practical due to their high voltage/current stresses. Instead, a buck+boost converter can be obtained by the cascade connection of two basic converters: a step-down converter, or "buck circuit," followed by a step-up converter or "boost circuit."
A conventional method of controlling the buck+boost converter is to operate the converter either in a buck or boost mode, depending on whether the input voltage is above or below the required output voltage. The buck+boost converter is configured to operate as a buck or boost converter by selectively controlling the operation of switches within the buck+boost converter's circuit. Operation of the buck+boost converter in the buck mode is accomplished by keeping a switch in the "boost circuit" in its non-conducting, or OFF, state and switching a switch in the "buck circuit," using pulse-width-modulation (PWM) control, when the input voltage is above the required output voltage. Operation of the buck+boost converter in the boost mode, on the other hand, is achieved by keeping the buck circuit's switch ON, i.e., conducting, at all times and controlling the switching of the boost circuit's switch, with PWM control, to regulate the output voltage.
However, with the above-described control scheme, if the input voltage is close to the output voltage, control accuracy degrades. Specifically, in the mode transition region where the converter's input and output voltages are about the same value, the control scheme cannot regulate the output voltage smoothly.
Accordingly, what is needed in the art is an improved control method for buck+boost converters that mitigates the above-described limitations and, more particularly, offers enhanced control accuracy in the mode transition region.